HIGH ALTITUDE PHYSIOLOGY

(Lecture delivered during the International Mountaineers' Meet, May 1973)

SURG. CAPT. M. S. MALHOTRA, I.N.

THE mountains are beautiful but the environments around them pose a constant hazard and challenge to the climbers who scale them. To meet this challenge, scientists all over the world are working towards understanding and solving the problems associated with it. We, in the Defence Institute of Physiology and Allied Sciences, have done extensive studies on various physiological problems pertaining to living and working at high altitude. I do hope these will be of use to you all.

There are two main problems which pose a severe hazard to the living and working of a mountaineer at high altitude—one is a lack of oxygen and the other is extreme cold.

The Lack of Oxygen

The air around us contains 79% nitrogen and 21% oxygen at all places, whether it is sea level or altitude. However, the pressure that the atmospheric air exerts varies with the altitude. At sea level, the atmospheric pressure is 760 mm Hg, of which about 21%, i.e., 159 mm Hg is due to oxygen and the remainder is nitrogen. There is a progressive fall in the atmospheric pre- sure with altitude, thereby reducing the partial pressure of oxygen. At an altitude of 3500 m. or 11,500 ft, the atmospheric pressure is 495 mm Hg and the partial pressure of oxygen 104 mm Hg. With higher altitudes, both these show increasing drop (see Table I Page 150).

The partial pressure of oxygen in the lung alveoli is always considerably less than that in the atmospheric air. At sea level, this pressure is 102 mm Hg as against 159 mm Hg in the atmospheric air, a difference of 57 mm Hg. The gap between these two pressures gets reduced to some extent with altitude clue to increase in ventilation of the lungs. If it is possible to have the same partial pressure of oxygen in the lung alveoli as in the atmospheric air, the mountains would pose no problem due to lack of oxygen.

Table 1-Oxyzen Tension in Air and Alveoli of the lunge at various altitudes and ambient temperature

The fall in the oxygen tension in the lung alveoli with altitude, in turn, affects the quantity of oxygen carried by the blood from the lungs to the rest of the body. In the lungs, there is exchange of gases, oxygen from the alveoli passes to the blood and C0₂ from the blood passes to the alveoli. The amount of oxygen that passes from the lungs to the blood, apart from other factors, is very much dependent upon the pressure of oxygen in the lung alveoli. This is because most of the oxygen (98%) is carried by the haemoglobin in the blood in loose combination which depends upon the pressure of oxygen in the lung alveoli. The amount of oxygen that combines with haemoglobin at various pressures follows an 'S' curve shown in Fig 1.

From this figure, it will be seen that the haemoglobin is 100% saturated with oxygen when the oxygen tension in the alveoli varies between 80-100 mm Hg. At an altitude of about 2720 nr., when the oxygen tension in the lungs is about 65 mm Hg, the haemoglobin is only 95% saturated, and the resultant 5% deficiency is not felt appreciably by the body. Beyond this altitude, the oxygen combining power of haemoglobin falls steeply, with the result that the amount of oxygen carried by the blood remains short of the requirement, leading to symptoms of oxygen lack such as headache, loss of appetite, lack of sleep and breath- lessness on exertion. These symptoms disappear in a few days as the body starts getting acclimatized to the changing environment. But, even after full acclimatization and stay of many months at high altitude, the oxygen uptake capacity remains much lower than in the plains. Let us see how one can best acclimatize to altitude and what changes take place in the body during this acclimatization process.

Recommended Acclimatization Procedure

Experience has shown that a high state of physical fitness and acclimatization to altitude is best achieved by making the ascent in stages. For this, the mountaineers should march slowly up to about 10,000 ft. and then the acclimatization routine should start. For this purpose, they should work at about 2000—3,000 ft. higher than the altitude at which they sleep. For example, on arrival at 10,000 ft., they should climb to about 12,000 ft. during the day and return to camp at 10,000 ft. to sleep at night. On the third day, they should move the camp to 12,000 ft. and work during the day at about 14,000 ft. and return to 12,000 ft. for sleep. They should keep the camp here for 2-3 days before moving higher by about 2,000 ft. The number of days spent in various stages goes on increasing with altitude, so that about 8—10 days are required to reach 18,000 ft. This practice of working at higher and sleeping at lower altitudes is adopted because the tolerance of an individual to high altitude is reduced during sleep. This is because firstly, the activity of the respiratory centre is depressed during sleep and secondly, the horizontal lying position raises the level of the diaphragm (i.e. the partition between the chest and the abdomen). Both these factors hinder the proper ventilation of the lungs and thereby cause a considerable lowering of the oxygen tension in the blood. This in turn leads to the inability to sleep and may cause other symptoms of oxygen lack. It has been estimated that sleep alone increases the effective altitude by about 2,000 ft.—in other words the depressive effects of sleep are equal to the elevation in altitude by about 2,000 ft.

Physiological Responses during Acclimatization

Let us see the physiological and biochemical changes in residents of plains during a stay of two years at an altitude of 4000 m.

(a) Changes in respiration

A person adapts to altitude by increasing both the rate and depth of respiration so that he breathes more air per minute than in the plains (Fig. 2).

This increase in minute ventilation occurs immediately on arrival and continues throughout the stay at altitude aid by better aeration of the lungs, helps to increase the oxygen tension in the alveoli by 4-5 mm Hg. This, in turn, results in greater transport of oxygen by haemoglobin of the blood. On return to sea level, the respiration again returns to its original lower value. The increased ventilation at high altitude is beneficial in protecting the lungs against diseases like tuberculosis.

(b) Changes in cardiovascular system

At altitude, the pulse rate increased by about 20-25 beats per minute soon after arrival and continues to be high for about one month. Thereafter, it falls with stay but still remains higher by 10-15 beats per minute than the original sea level values (Fig. 3).

However, on return to the plains after a stay of two years or so at altitude, the pulse rate falls to a much lower level than the previous value. Low pulse rate is normally seen in athletes, players, wrestlers who are physically strong. This shows that the efficiency of the heart is considerably improved at high altitude.

The blood pressure, both systolic and diastolic, increases by 5-10 mm Hg during the first week of arrival at high altitude. Thereafter it shows a progressive, though slight, increase till 10 months, after which systolic blood pressure stabilizes but diastolic continues to rise steadily but slowly. The maximum rise at 10 months is, by about 10-15 mm Hg in both systolic and diastolic pressures. Because of this, it is not advisable for people suffering from high blood pressure to take to high altitude mountaineering.

(c) Change in blood

The haemoglobin and red blood cells in the blood increase. This increase starts to occur within the first week of arrival at high altitude and reaches its maximum after about 10 months (Fig. 4).

Thereafter, both these show a slow continuous drop but still remain higher than the sea level values. The rise in haemoglobin results in increase in oxygen-carrying capacity of the blood and partially compensates for the loss in oxygen tension in the atmosphere. A comparative decline after 10 months shows that other adaptive changes, probably at cell level, have taken place in the body and the necessity for very high levels of haemoglobin is no longer there.

(d) Changes in blood choloesterol level

It has been found that scrum cholesterol begins to fall at altitude after about a month and continues to drop with further stay. This low blood cholesterol level is maintained for a few months even after return to the plains (Fig 5) .

As you know, the cholesterol level is closely associated with the incidence of coronary heart disease and in susceptible persons, efforts are made to reduce its level by means of dietary control or drugs. High altitude, by reducing the blood cholesterol, provides a natural protection against coronary heart disease and this effect persists for sometime even after return to the plains. It is very heartening to state that nowhere in the world, has any case of coronary heart disease been reported amongst the local residents of high altitude.

(e) Changes in blood sugar level

The other important change is in the fasting blood sugar level which starts rising after about a month of stay at high altitude. The maximum rise is seen after 10 months when the fasting blood sugar may reach 120 mg% as compared to 80 mg% on arrival (Fig. 5) . However, the utilisation of sugar is not disturbed, which shows that there is no reduction in the insulin production or its utilisation in the body, but the control mechanism of sugar is re-set at a higher level. This probably is adaptive in nature as, of all the foods, the body can utilise glucose most easily and it gives maximum energy for the same amount of oxygen utilised. Higher level of glucose in the blood, makes more of it available to the tissues for providing energy. This would, however, adversely affect the diabetes of actual or potential diabetics, who should avoid high altitude mountaineering.

(f) Changes in Work Capacity

The work capacity at high altitude is considerably reduced, depending upon the altitude. At 3500 m. for example, it is only 75% on arrival, but as one gets acclimatised by staying at altitudes, the physical work capacity improves. It is 81% after 1 month, 83% after 10 months and 86% after 14 months and is maintained at this level thereafter (Fig. 6).

This shows that for complete acclimitization to occur, it takes almost 14 months to stay at altitude. Only some of the changes that take place during this acclimatization process have been discussed. There are many more at the tissue level and all of these help to increase the oxygen-carrying and utilisation power of the body. On return to the plains these changes do not reverse to original sea-level values immediately but take sometime, with the result that for the first 2-3 weeks on return from high altitude, the physical efficiency in the plains is 5-7% higher than the initial value.

As some of the physiological changes during acclimatization may be detrimental for some individuals, it is desirable to screen out such persons during selection of the mountaineers for high altitude climbs.

Those with a past history of certain illness are not advised to go high in the mountains because at high altitude either their disease is likely to get aggravated or their acclimatization may be considerably delayed. These diseases are:

1. Obesity
2. High blood pressure
3. Anaemia, i.e., deficiency of blood
4. Peptic ulcer—these tend to bleed profusely
5. Liver diseases, like jaundice, amoebic hepatitis
6. Lung diseases, like chronic bronchitis, asthma, emphysema, tuberculosis
7. Heart disease
8. Diabetes
9. Nervous temperament

Those who are free from diseases usually stand the altitude well irrespective of age. There are, however, a certain number of absolutely healthy individuals who do not get acclimatized to altitude and are a handicap to the mountaineering team. In such persons, the symptoms of acute mountain sickness like vomiting, loss of appetite, inability to sleep, severe headache, persist for a long time without any improvement even after two to three weeks. These symptoms are more among the sedentary persons than in physically active persons. A test for selecting mountaineers on the basis of their physical fitness has been tried.

This is based on the scores of a step test which consists of stepping up and down a 15 inch (38 cm) high stool at 30 times per minute for 5 minutes. Their recovery pulse rate is counted after the exercise, at intervals of 1-1 ½ min, 2-2 ½ min and 3-3 1/2 min and the sum of these three pulses is taken to be the score. Lower the recovery pulse rate, higher is the physical efficiency. This test has been found to be very highly correlated with the performance of the students in various mountaineering courses.

Effects of altitude higher than 16,000 ft.

It has been seen that lowlanders can adapt to an altitude of 16,000 ft. and can stay there for 2-3 years without any ill-effect. But at high altitudes, it is not possible for them to adapt fully as further increase in their red blood cells and haemoglobin result in corresponding increase in the viscosity or thickness of the blood and makes its circulation difficult. Additional changes in respiration and circulation are also not enough to adapt to the new situation. Therefore, a person loses his body weight and physical efficiency if he stays longer than 2-3 months at 17,000—18,000 ft. This duration goes on reducing with still higher altitudes. At 25,000 ft, it is safe to stay only for a period of 7-10 days after which muscle degeneration and weakness start to occur and it would take more than 5-6 months for recovery on return to the plains. Accordingly, the Base Camps during various mountaineering expeditions should be kept below 17,000-18,000 ft. as far as possible.

Pulmonary oedema and Acute Mountain Sickness

There is a wide variation between various individuals in tolerance to high altitude. Quite a few of the mountaineers suffer from symptoms of acute mountain sickness consisting of headache, loss of appetite and loss of sleep. There are others who suffer from a serious type of disease, called 'Pulmonary Oedema' which proves fatal unless the individual is removed from high altitude immediately. Acute pulmonary oedema has been seen to occur more often in those persons who are re-entrants to altitude rather than the newcomers. Its incidence is more on mountains where there is less rain. The incidence of pulmonary oedema is 15% in the eastern Himalaya as compared to 8% in the western Himalaya which are comparatively dry having very little rainfall. This shows that climatic factors are important in its causation. The studies so far conducted have shown that those who do not pass a normal amount of urine during their first few days of arrival at high altitude, become victims of this disease. The mechanism of causation of oliguria (less urine) in susceptible persons is being investigated by us. We have been trying various drugs to prevent the occurrence of this disease at high altitude. Turosemide' has been found to be very effective for this purpose. One tablet (40 mg) is taken within 4 hours of arrival at an altitude above 10,000 ft and is repeated daily for next 7 days, which is the most susceptible period for occurrence of pulmonary oedema. Those who show symptoms of giddiness, muscle cramps or general weakness on a dose of one tablet daily, are given half a tablet daily (20 mg) and those who do not pass sufficient urine with one tablet, are given two tablets per day. Administration of this drug has helped to reduce the incidence of pulmonary oedema to a great extent.

For prevention against acute mountain sickness, many drugs have been tried. The most practical and effective one is Aspinn which, if taken a few hours before arrival at high altitude, is very useful in getting rid of a headache which can otherwise be very incapacitating.

Water Requirement

Previously, it was believed that dehydration was responsible for s(?me of the psychological symptoms like hallucination at high altitude. Our studies have shown that even when the body is dehydrated up to 5% of the body weight, no symptoms of hallucination or of any other psychological nature appear and the performance is not affected considerably. Requirement of water at high altitude has also been studied by us and it has been found that fluid requirements at high altitude are not more than those in temperate climates. Fluid intake of 1.5 to 2 litres per day has been found to be adequate. Symptoms of hallucination observed in certain mountaineers are therefore due to lack of oxygen supply to the brain rather than the lack of water.

Extreme Cold

The second problem at high altitude is that of extreme cold. The environmental temperature goes on reducing as we climb higher and higher. The mean environmental temperature encountered at various altitudes is given in Table. 1. The effect of cold on the body, however, does not depend on the temperature alone, but is also associated with the speed of the wind. Usually, low temperature and high wind co-exist at high altitude. Their combined effect on man has been worked out scientifically and is referred to as ‘Wind-Chill Factor of Siple’ (Fig 7).

Reference to this figure will show that frostbite is likely to occur after an exposuure of one hour to a temperature of -}-20^oF (—6.7°C) when it is associated with a blizzard of 40 mph or to a temperature of —40°F (—40°C) if the wind velocity is only 2 mph. Still air at a temperature of even—80° F (—62.2°C) may not pose any risk of frostbite. The main problem for the mountaineers in this regard is prevention of cold injury like frostbite. Let us see how does this occur and what are the preventive measures.

Frostbite

Frostbite has been responsible for the loss of many a toe and finger of a mountaineer. This injury occurs mostly in hands and feet when the environmental temperature is below 18°F (—7.8°C) and the duration of exposure to cold exceeds 2-3 hours. A person is more prone to get this injury if he is inactive during exposure. Frostbite of the hands is commonly associated with the wetting of the limbs or gloves. This is because water conducts heat 23 times faster than air. Elderly persons are more prone to get cold injury because of poor circulation in their limbs due to narrowing of their blood vessels. At high altitude, hypoxia makes a person more prone to get cold injury as it has the same effect on blood vessels as cold, in that it causes constrictions of blood vessels of the limbs and results in greater diversion of the blood to the internal organs. In frostbite, there is a freezing of skin and deeper tissues. The affected part becomes pale and lustreless. In a day or so, large blisters appear and after another 3-4 days the part becomes black and either drops off on its own in about two to three weeks or has to be surgically amputated. In superficial type of frostbite, only the skin and subcutaneous tissues are involved, while in the severe type deeper tissues like muscles and bones die and are cast off. It can involve one or two fingers or even the whole hand or foot depending upon the exposure.

Preventive Measures

It is possible to prevent cold injuries by observing certain precautions. A good health, adequate nutrition and adequate clothing are essential for protecting against cold injury. As much of the body as possible, should be protected against cold and wind by suitable clothing which should be loose and worn in layers. The inner layer should be of insulating material like wool and the outermost layer must be wind and waterproof. The clothes should be loose enough to allow adequate ventilation and evaporation of sweat. Damp and wet clothing should be replaced, otherwise the moisture in them will freeze, and will cool the body. The feet should have at least two pairs of socks and boots should have insulated rubber soles. The hands should be protected by leather gloves or mittens. The gloves should be tied at the wrist by a strap to avoid ingress of snow and water. The sleeves of the outer coat should cover the end of the gloves and be strapped so that snow flakes cannot find their way into the gloves. However, strapping should not be so tight as to produce a constriction effect on the wrist. Socks should be well donned, as badly-donned socks are liable to injure the feet. Leather boots, if used, should be properly fitted and be kept soft by application of special dubbin. When working in snow and slush, the bottom of the trousers should be left outside the upper end of the boots to prevent ingress of snow and slush.

Direct contact with cold metallic objects should be avoided, as they conduct heat very fast. If any cold any cold metallic article is handled, it is likely to result in cold injury of the hand and fingers. When it is necessary to handle such objects, always use gloved hands. Cold metallic objects like frames of spectacles should be wrapped with tape. As wood does not conduct heat quickly, the items of furniture and handles of metallic gadgets and equipment should be made of wood or any other similar insulating material.

One should remain active, move one's hands and feet, and wriggle the facial muscles frequently during exposure to cold. This helps not only in the production of heat but also in the circulation of the blood to the exposed parts. If one is required to stand for long periods, it is better to move the toes inside the boots. Sleeping with boots on should be avoided as it would impede the blood circulation through the feet. It is better to keep the boots inside the sleeping bag at night so that these are kept warm for the morning wear.

It is essential to maintain a good nutrition and the meals taken should be hot. Food is the only source of energy for the body and its supply must remain adequate. The diet should contain plenty of carbohydrates, proteins and fats. Use of alcohol should be avoided before going out into the cold as it increases the blood supply to the skin and limbs which in turn result in greater heat loss from the body.

One should keep clean, but frequent bathing in cold weather is not recommended as this washes off the protective layer of grease on the body which acts as a good insulating material. The nails should be kept trimmed.

First Aid treatment

If frostbite lias occurred, first aid treatment should be given. All constricting items like boots, gloves and socks should be removed. Thereafter re-warm the part in lukewarm water and then ensure that it is kept warm. If warm water is not available, then place the part in contact with the warm abdomen or hold it in warm hands. Do not re-warm by rubbing, massage or exposure to an open fire. It is necessary to keep the general body warm, by use of blankets and hot drinks. If there is any big blister, it should be covered with some dry dressing. The patient should not be given any cigarettes to smoke or alcoholic drink. He should be treated as a stretcher case.

 

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